riscv64: fix debuggerd_test build.

This adds the missing assembler for riscv64, even though I don't have a
working tombstoned yet to test it with. There's a distinct possibility
we'll be back to fix the test (because although "register 1" is harmless
for the other architectures, it's the ra register on riscv64; the default
link register), but at least this lets us build the test.

I've also simplified all the assembly to be the simplest sequence I
know that writes 0 to address 0 (because if there was a reason to use
so many instructions before, I want to know what it is so I can write
the missing comment!).

Test: treehugger
Change-Id: I10d117eaedf361d9759a450e0973d07c4f97090e
diff --git a/debuggerd/Android.bp b/debuggerd/Android.bp
index 5da1b40..d09498f 100644
--- a/debuggerd/Android.bp
+++ b/debuggerd/Android.bp
@@ -509,6 +509,9 @@
         arm64: {
             src: "seccomp_policy/crash_dump.arm.policy",
         },
+        riscv64: {
+            enabled: false,
+        },
         x86: {
             src: "seccomp_policy/crash_dump.x86_64.policy",
         },